Trigger circuit



United Sttes Patent 3,084,282 TRIGGER CIRCUIT George L. Clark, Hawthorne, and John J. Hickey, Lawndale, Calif., assignors to Space Technology Laboratories, Inc., Hawthorne, Calif., a corporation of Delaware Filed Jan. 3, 1961, Ser. No. 89,385 8 Claims. (Cl. 315-168) This invention relates to trigger circuits utilizing grid controlled thyratrons and more particularly to improvements directed towards increasing the switching speed of such circuits.

Grid controlled miniature thyratrons have been found useful in pulse forming circuits because of their large current carrying capabilities and fast switching characteristics. One form of thyratron, or gas discharge device, may comprise a cathode, an anode, a shield electrode, and a trigger electrode or grid enclosed in a gas filled envelope. Normally, the tube is held in a nonconducting or off condition by biasing the grid negatively (below a certain threshold potential known as the firing voltage), with the anode being maintained at a highly positive potential relative to the cathode and shield electrode. The firing voltage is that grid potential, which if exceeded will cause the tube to conduct, and which if not attained, will cause the tube to remain nonconducting. The tube may be triggered on, to a con-ducting condition, by applying a positive trigger voltage to the grid which drives the grid beyond the firing voltage by an amount known as the grid overvoltage.

For applications involving the study of high speed transient phenomena, such as plasma shock waves, it is very important that the switching time of thyratrons be reduced to a minimum. By the application of a very large grid overvoltage and a maximum anode voltage, the delay time between the time of application of the grid trigger pulse and the time for the anode current to reach approximately of its maximum value, can be reduced ultimately to about 50 nanoseconds (1 nanosecond equals 1X10" seconds). However, it is desirable that still higher switching speeds be attained.

Another problem that is encountered has to do with the variation in the delay time, more commonly referred to as jitter.

---Furthermore, an undesirable effect resulting from the use of a, highly positive grid pulse to trigger the tube is the drawing of current by the grid and the requirement for a high current trigger source to supply this grid current. Moveover, if the output is taken across a cathode load resistor, the grid current distorts the shape of the output pulse.

Accordingly, it is a principal object of this invention to reduce the switching time of thyratrons.

Another object is to reduce the jitter time between successive firings of a thyratron tube.

A further object of this invention is the provision of a thyratron trigger circuit in which substantially no grid current is drawn.

The foregoing and other objects are realized through the use of a circuit including a thyratron in which two electrodes are interposed between the cathode and anode. The electrode closest to the cathode functions as a control electrode and is biased sufficiently negative with respect to the cathode to maintain the tube normally nonconducting. The other electrode, which is positioned between the control electrode and anode, functions as an accelerating electrode and is biased very highly positive with respect to the cathode.

In order to trigger the tube to a conducting condition a positive trigger voltage pulse equal in amplitude and opposite in polarity to the control electrode bias is applied to the control electrode. The trigger pulse reduces the control electrode voltage to zero, thereby instantaneously exposing the electrons in the vicinity of the cathode to the full accelerating electrode potential and permitting a more rapid ionization of the gas molecules.

In contrast to prior art trigger circuits in which a minimum switching time value of 50 nanoseconds is attainable, the present trigger circuit has successfully achieved much lower switching times of about 10 nanoseconds.

Further, since the control electrode is not driven posi-. tive by the trigger pulse, the control electrode circuit does not draw current and the-current supply requirements of the trigger source are reduced to a minimum.

In the drawing:

' FIG. 1 is a block diagram of an electronic camera system in which the improved thyratron trigger circuit of the invention finds a particular utility;

FIG. 2 is a perspective view, partly broken away, showing the construction of a thyratron tube of the kind used in the circuit of the invention;

, FIG. 3 is a section taken along line 3-3 of FIG. 2; and

FIG. 4 is a schematic circuit of a thyratron trigger circuit according to the invention. 7 1 I Referring now to the drawings in which like numerals refer to similar parts, FIG. 1 is a block diagram of an electronic camera system in which the improved thyratron trigger circuit of the invention finds particular utility; The electronic camera system includes as one of its principal components an image converter tube 10 which functions primarily as a high speed shutter. Another function of the'image converter tube 10' is that of providing light amplification for-the extremely short frame times involved in its high speed photographic operation.

The image converter tube 10 comprises essentially a cylindrical evacuated envelope 12 containing a photoemissive cathode or photo cathode 14 at one end, a fluorescent screen 16 at the other end, a control grid 18 adjacent to the photo cathode 14, and a pair of deflection plates 20 and 22 intermediate the control grid 18 and fluorescent screen 16. Certain other parts and compo nents essential to the operation of the tube 10 are omitted for simplicity, since these are well known. For example, the tube 10 ordinarily contains additional electrodes such as an anode and focusing electrodes and also requires a high voltage supply. It will suffice to say that the tube may be one of the kind manufactured by RCA and hear: ing the developmental type number C73435A. It should be kept in mind that the voltage relationship of the elements are critical if a sharp focusing is desired.

It will be apparent that with an object 24 such as gas, heated to a temperature of millions of degrees, for a period of a few nanoseconds, the problem of obtaining desired data is acute. In the operation of the electroniccamera for the purpose of photographing high speed transient phenomena, light from an object 24 is focused by a lens 26 onto the photoemissive cathode 14 of the image converter tube 10. The electron image emitted from the photo cathode 14 is normally prevented from reaching the fluorescent screen 16 by the application of a sufiiciently high negative blanking voltage to the control grid 18 relative to the photocathode 14. g

In operation, a rapid series of frames or exposures of the phenomena or object 24 can be taken by applying a series of rectangular gating voltage pulses to the control grid 18. The gating voltage pulses are sufficiently large such as 300 volts, to unblank the control grid 18 and permit the electron image to be accelerated towards the fluorescent screen 16. The different frames or exposures may be reproduced side-by-side on the fluorescent screen by applying deflection voltages to the deflection plates 20 3 and 22 respectively, between and during successive gating pulses. The amplified light images appearing on the fluorescent screen are then projected onto a photographic film 28 by means of a lens system 30. In practice the film 28 may be part of'a camera of the type which allows rapid development of the exposed film 28.' i

A trigger signal for actuating the electronic camera tube is developed in a circuit which includes a photosensitive cellQ lexpose'd through a lens system 34 to the phenome- -"f1fdh1 ,o r object 24 to be recorded. The beginning of the event for example,.may be manifested by the initial emission of light from the object 24. The light emission is picked up by the cell 32 where it'is converted into an electrical impulse. The electrical impulseisamplifiedin an electrical amplifier stage 36 and the amplified impulse is fed to a thyratron trigger circuit 38 where it is further amplified and used to trigger a gating pulse generator 40 and a deflection pulse generator 42 to generate the gating and deflection pulses which are fed to the camera tube 10. Since the event under scrutiny itself is used to actuate the camera tube '10, it will be seen that the time delay developed in the thyratron trigger circuit 38 must be kept as short as possible, so as not to lose an appreciable part of the event. In accordance with the invention, the time delay occurring in the thyratron trigger circuit 38 is minimizedby means of'an' improved thyratron switching circuit.

' FIGS. 2 and 3 illustrate the construction of a thyratron tube 44 of the kind used in the trigger circuit of the invention. The tube 44 comprises an envelope 46 containing a filling of an ionizable gas, such as hydrogen or argon. Within the envelope 46 are mounted a series of electrodes including an electron source, such as a thermionic cathode 48, an anode 50 spaced therefrom, an accelerating electrode 52 intermediate the cathode 48 and anode 50, and

a control electrode 54 surrounding the electrodes 48, 50,

and 52.

The control electrode 54 is provided with two beamforming members or apertured walls 56 and 58. One wall 56 extends laterally of the path between the cathode 48 and the accelerating electrode 52. The second wall 58 extends laterally of the path between the accelerating electrode 52 and the anode 50. The two apertured walls 56 and 58 serve to define a single path along which the gas in the tube can break down. In the absence of the path-defining walls 56 and 58, there would be a multiplicity of possible ionization paths, which could result in a multiplicity of difierent firing times and firing 'voltages, thereby giving rise to excessive jitter. In addition, the walls 56 and 58 serve to confine the electrons and ions flowing between the cathode 48 andthe anode 50 to a narrow beam. More importantly, the first wall 56 is mounted relatively close to the cathode 48 so that it can exert a high degree of control over the electron flow and thereby control the ionization time of the gas within the envelope 46.

Each of the four electrodes 48, 50, S2, and 54 may have an elongated shape, with the cathode 48 cylindrical, oval, or flat, the accelerating electrode 52 preferably flat with an elongated slit therein, the anode 50 flat, and the controlflelectrode 54 a rectangular box-like structure.

As discussed above, for many applications, it is quite important that the thyratron tube 44 be capable of being triggered to a conducting condition in as short a time as possible. Toward this end, the tube 44 is arranged .in a circuit, with operating potentials being applied such as to cause the tube 44 to conduct immediately upon the application of a trigger voltage pulse. p r

The tube 44 is shown arranged in a trigger circuit in FIG. 4. As shown, the cathode 48 is connected to ground through a cathode resistor 60. The accelerating electrode 52 is maintained at a relatively high positive potential by connection to one tap 62 of a voltage divider network which includes ,three resistors 64, 66, and 68 connected across a, direct current high voltage supply 70. For

example, the accelerating elecetrode 5 2 is connected between the two resistors 64 and 66. The anode 50 is main tained at a higher positive potential than the accelerating electrode 52 by connection to the other tap 72 or junction between the two resistors 66 and 68 of the voltage divider. The control electrode 54 is negatively biased somewhat beyond cutofi voltage by connection to a negative bias source 74 through a grid resistor 76. The positive input trigger pulse is applied to the control electrode 54 through a direct current blocking capacitor 78. The output pulse is taken across the cathode load resistor 60.

An energy storage means, such as a capacitor 80 and a resistor 82 is connected in a first branch in series with the tube 44. The high voltage supply 70 forms part of a second branch in series with the tube 44; During the nonconducting condition of the tube 44, the capacitor 80 is charged to the full voltage of the voltage supply 70 which is in a closed circuit with the capacitor 80. Subsequently, when the tube 44 is triggered to its conducting condition, the capacitor 80 is discharged through the cir cuit including the tube 44, the cathode load resistor 60, and the resistor 82. The flow of discharge current through the cathode load resistor 60 produces a voltage pulse across the latter.

The potentials applied to the accelerating electrode 52 and the anode 50 are of a magnitude just short of their respective break-down voltages. Typically, the anode 50 potential may be 1000 to 3000 volts positive and the accelerating electrode 52 potential may be 100i) to 2000 volts positive. Under these conditions, the control electrode 54 potential may be 100 to 150 volts negative.

Only a relatively low negative bias is required on the control electrode 54 to maintain the tube 44 normally nonconducting because of the close proximity of the control electrode 54 to the cathode 48. By the same token, only a relatively small positive trigger pulse is required on the control electrode 54 to switch the tube 44 into a conducting condition.

With the normal operating voltages applied to the ,thyratron tube 44, the electrons emitted from the cathode 48 are repelled [by the negatively biased control electrode 54 and thus they can not reach a velocity suffic'ient to ionize the gas within the tube 44. When the trigger pulse is applied to the control electrode 54, the voltage on the control electrode 54 is quickly raised beyond cutoff and brought to the cathode 48 potential. Any electrons emitted thereafter are exposed to the full high potential of the accelerating electrode 52 and thus are immediately brought to the velocity of ionization of the gas molecules. In this Way, the tube 44 is quickly switched to a conducting condition.

In contrast to this, were the thyratron tube 44 normally kept nonconducting by biasing the accelerating electrode 52 slightly negatively, with the control electrode 54 at cathode potential, and the tube 44 then switched by applying to the :acelerating electrode52 a highly positive trigger pulse, as is done in some prior art trigger circuits, the accelerating electrode 52 would have to he driven positive and thus would draw current. The drawing of electrode current 'avould cause a slow 'rise in the voltage on the accelerating electrode 52. Consequently, there would be some delay before the electrons would be exposed to a high enough accelerating Voltage to cause ionization.

The present trigger circuit has successfiully reduced the switching time to about 10 nanoseconds. In addition, the jitter time has been reduced to less than .1 nanosecond as compared to a value of not less than 1 nanosecond for the prior art. Also, because of the greater control exerted by the control electrodev 54 relaxtive'to the acceleratingv electrode 52 over the flow of electrons, coupled the :fact' that the control electrode 54 is never driven positive, a substantial reduction. in the-power requirements of the trigger pulse source has been achieved. Furthermore, the output taken across the cathode load resistor 60 is undistorted, which is not the case when the triggered electrode draws current.

What is claimed is:

1. In combination: a gas discharge device including an envelope containing an ionizable gas, and within said envelope an electron source, an anode spaced from said electron source, a beam forming control electrode, at least portions of which lie between said electron source and said anode, an accelerating electrode between said control electrode and said anode; means connected to apply positive operating potentials to said anode and said accelerating electrode relative to said electron source; and means connected to apply a negative operating potential to said control electrode relative to said electron source, said negative potential being suflicient to maintain said discharge device in a normally nonconducting state.

2. The combination according to claim 1, wherein the potentials applied to said accelerating electrode and said anode are of the order of ten times the negative potential applied :to said control electrode.

3. In combination: a gas discharge device including an envelope containing an ionizable gas, and within said envelope an electron source, an anode spaced from said electron source, a control electrode, at least portions of which lie between said electron source and said anode, an accelerating electrode between said control electrode and said anode; means connected to apply positive operating potentials to said anode and said accelerating electrode relative to said electron source; means connected to apply a negative operating potential to said control electrode relative to said electron source, said negative potential being sufiicient to maintain said discharge device in a normally nonconducting state; and means connected to apply to said control electrode a positive trigger voltage of suflicient magnitude to drive said discharge device into a conducting state but insufiicient to drive said control electrode positive.

4. The combination according to claim 3, wherein said trigger voltage is substantially equal in magnitude to the operating potential applied to said control electrode.

5. In combination: a gas disch "ge device including an envelope containing an ionizable gas, and within said envelope a cathode, an anode spaced from said cathode, a control electrode between said cathode and said anode, an accelerating electrode between said control electrode and said anode; means connected to apply positive potentials to said anode and said accelerating electrode relative to said cathode; means connected to apply a negative bias potential to said control electrode relative to said cathode, said negative bias potential being sufiicient to maintain said discharge device in a nonconducting state; means connected to apply to said control electrode a positive trigger voltage of magnitude substantially equal to but not greater than said negative bias potential, [thereby to drive 6 said discharge device into :a conducting state; an output terminal connected to said cathode; and a cathode load resistor connected in series with said output terminal and said anode.

6. In combination: a gas discharge device including an envelope containing an ionizable gas, and within said envelope a cathode, an anode spaced from said cathode, an accelerating electrode between said cathode and said anode, a control electrode surrounding said cathode, accelerating electrode and anode, and having a portion thereof extending between said anode and said cathode and accelerating electrode and another portion thereof extending between said anode and said accelerating electrode; means connected to apply positive potentials to said anode and said accelerating electrode relative to said cathode; means connected to apply a negative operating potential 'to's'aid control electrode relative to said oath-i ode, said negative potential being suflicient to maintain said discharge device in a nonconducting state; and means connected to apply to said control electrode a positive trigger voltage of suflicient magnitude to drive said discharge device into a conducting state but insuflicient to drive said control electrode positive.

7. In combination: a gas discharge device including an envelope containing an ionizable gas, and within said envelope a cathode, an anode spaced from said cathode, a control electrode between said cathode and said anode, an accelerating electrode between said control electrode and said anode; means connected to apply positive potentials to said anode and said accelerating electrode relative to said cathode; means connected to apply a negative bias potential to said control electrode relative to said cathode, said negative bias potential being sufficient to maintain said discharge device in a nonconducting state; means connected to apply to said control electrode a positive trigger voltage of magnitude substantially equal to but not greater than said negative bias potential, thereby to drive said discharge device into a conducting state; an output terminal connected to said cathode; a cathode load resistor connected in series with said output terminal and said anode; an energy storage means connected in a first branch in series with said discharge device; and a direct current voltage supply connected in a second branch in series with said discharge device.

8. The combination according to claim 7, wherein said energy storage means includes a capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,772 Cone Apr. 23, 1946 2,516,675 Carne July 25, 1950 2,591,899 Webster Apr. 8, 1952 

1. IN COMBINATION: A GAS DISCHARGE DEVICE INCLUDING AN ENVELOPE CONTAINING AN IONIZABLE GAS, AND WITHIN SAID ENVELOPE AN ELECTRON SOURCE, AN ANODE SPACED FROM SAID ELECTRON SOURCE, A BEAM FORMING CONTROL ELECTRODE, AT LEAST PORTIONS OF WHICH LIE BETWEEN SAID ELECTRON SOURCE AND SAID ANODE, AN ACCELERATING ELECTRODE BETWEEN SAID CONTROL ELECTRODE AND SAID ANODE; MEANS CONNECTED TO 